1. The Prior Art
The most widely employed method for producing olefins is by the thermal cracking of hydrocarbons in tubular pyrolysis furnaces. However, as economic factors dictate that progressively heavier feedstocks be used for olefin or ethylene manufacture, two shortcomings in the conventional tubular pyrolysis furnace become more apparent. For one, coking rates both in the furnace tubes and in the furnace effluent quench area are greatly increased. In addition, substantial yields of light saturates and heavy cracked liquid by-products increase the capital cost of the downstream product handling facilities and decrease the total value of the yield spectrum.
An alternative approach to the use of tubular pyrolysis furnaces is use of molten bath heat carrier circulation systems. These molten bath heat carrier circulation systems typically are high temperature, low residence time systems which result in certain yield advantages and are of a relatively non-coking nature. See generally Fair et al., Commercial Ethylene Production by Propane Pyrolysis in a Molten Lead Bath, Chemical Engineering Process, Vol. 53, No. 9, pp 433-438 (1957). However, the presently available molten bath cracking systems are far from ideal.
The molten bath cracking system disclosed in Bruns, U.S. Pat. No. 2,931,843, comprises a reactor having a molten lead reservoir wherein the hydrocarbon feed is passed through the reservoir and the resulting cracked gases are quenched by cooled molten lead sprayed from the upper portion of the reactor. There are a number of disadvantages to the Bruns process and apparatus. In Bruns, the molten lead reservoir is relatively static while the hydrocarbon feed is bubbled through the bed. This type of operation at the required high superficial gas velocities leads to severe vibration, slugging and backmixing plus entrainment. The backmixing of reactants results in excessive coke make. Likewise, molten bed reactors employing similar "dip-tube" arrangements have identical problems. Still another serious problem in Bruns arises due to the erosion of the spray-type nozzles employed for quenching.
Another type of molten metal cracking apparatus is disclosed in Ozawa et al., U.S. Pat. No. 3,718,708, where solid grains or oils of high boiling point are employed for contacting purposes in the tubular quench section. This system has obvious disadvantages due to the complexity of the quench system.
Other types of molten medium cracking processes employ molten salts as the heat transfer medium. A representative molten salt process is disclosed in Hendal et al., U.S. Pat. No. 3,081,256. in Hendal et al., the molten salt is circulated by gas-lift action in a riser and is reheated by submerged combustion in a separate section of the reactor vessel. Cracked gases are separated from the molten salt at the top of the riser by a semi-centrifugal separator while coke formed in the reaction is burned off in the submerged regeneration section of the reactor vessel. Quenching of the cracked gas is accomplished by conventional means (steam contacting or heat exchange) downstream of the riser outlet salt separator. While the high heat transfer rates attained in the molten salt riser employed in Hendal et al. allowed shorter residence times and reaction temperatures than conventional tubular pyrolysis furnaces, the process as disclosed has two basic disadvantages. First, since salt is employed as the molten medium, steam or water are typically not employed in the reactor since steam reacts with the salt. Secondly, since the molten salt is not employed as the quench material, it is necessary to prevent molten salt loss from the reaction zone. This requires longer residence time in the separation chamber, therein permitting excessive coke formation and secondary reactions (resulting in the loss of valuable product) prior to quenching of the reaction products.
2. Summary of the Invention
A new process and apparatus for the thermal cracking of gaseous and liquid hydrocarbons is disclosed which improves the yield structure of the gaseous reaction products, reduces coke make, eliminates the problems associated with molten medium carryover in the reaction chamber, and permits the use of steam in the reaction chamber to lower partial pressure thus lowering coke made and improving selectivity to olefin.
In broad terms, the present invention is an improved process for the thermal cracking of hydrocarbons which process comprises:
1. heating a hydrocarbon feedstock to a temperature of between about 1400.degree. F and about 1900.degree. F in a reaction zone containing a forced circulation riser and in the presence of a molten metal;
2. separating the resulting gaseous reaction products from the molten metal by gravity separation;
3. circulating the molten metal to a reheat zone and then back to the reaction zone;
4. routing the gaseous reaction products from the reaction zone through a transition zone to a quench zone containing a forced circulation riser, wherein the residence time of the gaseous reaction products in the transition zone is less than 0.02 seconds;
5. cooling the gaseous reaction products to a temperature of between about 800.degree. F and about 1200.degree. F in the quench zone by contacting the gaseous reaction products with the molten metal;
6. separating the resulting gaseous reaction products from the molten metal by gravity separation; and
7. circulating the molten metal to a heat removal zone and then back to the quench zone.
Also disclosed is an apparatus for the high temperature thermal cracking of a liquid or gaseous hydrocarbon in the presence of a molten metal which comprises:
1. a reaction zone having
a. a vertical tubular reaction chamber which has an inlet for supplying a hydrocarbon feedstock, an inlet for supplying molten metal, and a reaction product outlet communicating with a hot product separation chamber, PA1 b. a hot product separation chamber communicating with the outlet of said reaction chamber having an outlet for separated reaction product and a liquid seal of molten metal communicating with a reheat chamber, and PA1 c. a reheat chamber which surrounds at least a major proportion of the reaction chamber, which reheat chamber has inlet and outlet means for passing molten metal from the hot product separation chamber to the reaction chamber and heating means for maintaining the temperature of the molten metal in the reaction zone at above 1400.degree. F; PA1 a. a vertical tubular cooling chamber communicating with the transition zone which has an inlet for the reaction product from the transition zone, an inlet for supplying molten metal, and a cooling chamber product outlet communicating with a cooled product separation chamber, PA1 b. a cooled product separation chamber communicating with the outlet of said cooling chamber having an outlet for separated cooled reaction product and a liquid seal of molten metal communicating with a heat removal chamber, and PA1 c. a heat removal chamber which surrounds at least a major proportion of the cooling chamber, which heat removal chamber has inlet and outlet means for passing molten metal from the cooled product separation chamber to the cooling chamber and cooling means for maintaining the temperature of the molten metal in the quench zone at between about 800.degree. F and about 1200.degree. F.
2. a transition zone communicating with the outlet of the hot product separation chamber of the reaction zone and communicating with the inlet of the cooling chamber of the quench zone; and
3. a quench zone having